Energy mixer

ABSTRACT

Disclosed is an energy mixer having a first active diode coupled between a first input node and an output node, and a second active diode coupled between a second input node and the output node. A first capacitor is coupled between the first input node and a dynamic node, and a second capacitor is coupled between the second input node and a third node. Switching circuitry is configured to selectively couple the dynamic node between a fixed voltage node and the second input node in response to a control signal provided by control circuitry. When an output voltage at the output node is within a first range, the dynamic node is coupled to the fixed voltage node and when the output voltage is within a lower voltage second range, the dynamic node is coupled to the second input node such that first capacitor and second capacitor are coupled in series.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No. 15/184,319, filed Jun. 16, 2016, now U.S Pat. No. 9,973,007, the disclosure of which is hereby incorporated herein by reference in its entirety.

FIELD OF THE DISCLOSURE

The present disclosure relates to energy harvesting systems that harvest energy from multiple energy sources.

BACKGROUND

Traditional energy harvesting systems are inefficient in mixing energy generated from multiple sources, because traditional energy harvesting systems do not prevent system startup cycle in situations of insufficient voltage generation from one or more of the multiple sources. Moreover, traditional energy harvesting systems typically use diode devices for energy mixing, resulting in substantially energy wasteful voltage drops. What is needed is an energy mixer that overcomes the problems of intermittent availability of all energy sources at the same time to avoid a full startup cycle triggered by energy source voltage output drop, and energy wasteful diode voltage drops.

SUMMARY

The present disclosure provides an energy mixer having a first active diode coupled between a first input node and an output node, and a second active diode coupled between a second input node and the output node. A first capacitor is coupled between the first input node and a dynamic node, and a second capacitor is coupled between the second input node and a third node. Switching circuitry is configured to selectively couple the dynamic node between a fixed voltage node and the second input node in response to a control signal provided by control circuitry. When an output voltage at the output node is within a first range, the dynamic node is coupled to the fixed voltage node; and when the output voltage is within a lower voltage second range, the dynamic node is coupled to the second input node such that first capacitor and the second capacitor are coupled in series.

In an exemplary embodiment, a first energy harvester has a first voltage output that is coupled to the first input node, and a second energy harvester has a second voltage output that is coupled to the second input node. The exemplary embodiment also includes a first buck converter with a first converter input coupled to the output node, and a first converter output for providing a first buck voltage. A second buck converter with a second converter output is coupled to the output node with a second converter output for providing a second buck voltage. Other exemplary embodiments include additional energy harvesters and active diodes.

Those skilled in the art will appreciate the scope of the present disclosure and realize additional aspects thereof after reading the following detailed description of the preferred embodiments in association with the accompanying drawing figures.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

The accompanying drawing figures incorporated in and forming a part of this specification illustrate several aspects of the disclosure, and together with the description serve to explain the principles of the disclosure.

FIG. 1 is an exemplary embodiment of an energy mixer configured to mix energy gathered from two different energy harvesters in accordance with the present disclosure.

FIG. 2 is another exemplary embodiment of the energy mixer that includes a third energy harvester.

FIG. 3 is a simulation graph of voltage stored by a capacitor that stores energy gathered by a piezoelectric harvester coupled to the energy mixer of FIGS. 1 and 2 when switching circuitry that prevents re-start is inactive.

FIG. 4 is a simulation graph of voltage stored by a capacitor that stores energy gathered by a thermoelectric generator harvester coupled to the energy mixer of FIGS. 1 and 2 when switching circuitry that prevents re-start is inactive.

FIG. 5 is a simulation graph of voltage at the output node of the energy mixer of FIGS. 1 and 2 when switching circuitry that prevents re-start is inactive.

FIG. 6 is a simulation graph of a control signal generated by the energy mixer of FIGS. 1 and 2 when switching circuitry that prevents re-start is inactive.

FIG. 7 is a simulation graph of the control signal generated by the energy mixer of FIGS. 1 and 2 when switching circuitry that prevents re-start is active.

FIG. 8 is a simulation graph of voltage stored by the capacitor that stores energy gathered by the piezoelectric harvester coupled to the energy mixer of FIGS. 1 and 2 when switching circuitry that prevents re-start is active.

FIG. 9 is a simulation graph of voltage stored by the capacitor that stores energy gathered by the thermoelectric generator harvester coupled to the energy mixer of FIGS. 1 and 2 when switching circuitry that prevents re-start is active.

FIG. 10 is a simulation graph of voltage at the output node of the energy mixer of FIGS. 1 and 2 when switching circuitry that prevents re-start is active.

DETAILED DESCRIPTION

The embodiments set forth below represent the necessary information to enable those skilled in the art to practice the embodiments and illustrate the best mode of practicing the embodiments. Upon reading the following description in light of the accompanying drawing figures, those skilled in the art will understand the concepts of the disclosure and will recognize applications of these concepts not particularly addressed herein. It should be understood that these concepts and applications fall within the scope of the disclosure and the accompanying claims.

It will be understood that, although the terms first, second, etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and, similarly, a second element could be termed a first element, without departing from the scope of the present disclosure. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items.

It will be understood that when an element such as a layer, region, or substrate is referred to as being “on” or extending “onto” another element, it can be directly on or extend directly onto the other element or intervening elements may also be present. In contrast, when an element is referred to as being “directly on” or extending “directly onto” another element, there are no intervening elements present. Likewise, it will be understood that when an element such as a layer, region, or substrate is referred to as being “over” or extending “over” another element, it can be directly over or extend directly over the other element or intervening elements may also be present. In contrast, when an element is referred to as being “directly over” or extending “directly over” another element, there are no intervening elements present. It will also be understood that when an element is referred to as being “connected” or “coupled” to another element, it can be directly connected or coupled to the other element or intervening elements may be present. In contrast, when an element is referred to as being “directly connected” or “directly coupled” to another element, there are no intervening elements present.

Relative terms such as “below” or “above” or “upper” or “lower” or “horizontal” or “vertical” may be used herein to describe a relationship of one element, layer, or region to another element, layer, or region as illustrated in the Figures. It will be understood that these terms and those discussed above are intended to encompass different orientations of the device in addition to the orientation depicted in the Figures.

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the disclosure. As used herein, the singular forms “a,” “an,” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises,” “comprising,” “includes,” and/or “including” when used herein specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.

Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. It will be further understood that terms used herein should be interpreted as having a meaning that is consistent with their meaning in the context of this specification and the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.

The present disclosure describes an energy mixer for multi-source energy harvesting. The disclosed energy mixer can be also be considered an energy combiner. FIG. 1 is an exemplary embodiment of an energy mixer 10 configured to mix energy gathered from a first energy harvester 12 and a second energy harvester 14 in accordance with the present disclosure. The energy mixer 10 includes a first active diode D_(ACT1) coupled between a first input node 16 and an output node 18, and a second active diode D_(ACT2) coupled between a second input node 20 and the output node 18. A first capacitor 22 is coupled between the first input node 16 and a dynamic node 24, and a second capacitor 26 is coupled between the second input node 20 and a third node 28. In this particular embodiment, the third node 28 is a fixed voltage node such as ground. The first capacitor 22 and the second capacitor 26 typically have capacitances in the microfarad range. However, it is to be understood that other capacitance ranges are within the scope of the present disclosure.

A first switching circuitry 30 is configured to selectively couple the dynamic node 24 between a fixed voltage node 32, typically ground, and the second input node 20 in response to a control signal ϕ₁. Control circuitry 34 is configured to provide the control signal ϕ₁ that has levels based upon two conditions. In a first condition, when an output voltage V_(MIX,OUT) at the output node 18 is in a first range, the dynamic node 24 is coupled to the fixed voltage node 32. In a second condition, when the output voltage V_(MIX,OUT) is in second range that is below the first range, the dynamic node 24 is coupled to the second input node 20 such that the first capacitor 22 and the second capacitor 26 are coupled in series.

In the exemplary embodiment of FIG. 1, the first energy harvester 12 has a first voltage output 36 coupled to the first input node 16, and the second energy harvester 14 has a second voltage output 38 coupled to the second input node 20. The second energy harvester 14 gathers a different form of energy than the first energy harvester 12.

In this example, the first energy harvester 12 includes a piezoelectric harvester 40 that generates an alternating current (AC) voltage that is converted into a direct current (DC) voltage by a rectifier 42. As depicted in FIG. 1, the piezoelectric harvester 40 is modeled by a source S1 and a capacitor C1, while the rectifier is modeled with traditional rectifying diodes D1 and D2.

The second energy harvester 14 includes a thermoelectric generator (TEG) harvester 44 and a DC-DC converter 46. Voltage generated by the TEG harvester 44 (i.e., V_(TEG),out) is boosted by the DC-DC converter 46, which includes an inductor-capacitor circuit L1-C2 that is switched by a first converter transistor M1 driven by a first clock signal CLK1 and a second converter transistor M2 driven by a second clock signal CLK2. In this exemplary embodiment, the first clock signal and the second clock signal are derived from a master clock signal CLK provided by a clock generator 48.

In one embodiment, the energy mixer 10 further includes a first buck converter 50 and a second buck converter 52. The first buck converter 50 has a first converter input 54 that is coupled to the output node 18 and a first converter output 56 for providing a first buck voltage V_(OUTH). The second buck converter 52 has a second converter input 58 that is coupled to the output node 18 and a second converter output 60 for providing a second buck voltage V_(OUTL) that is lower in voltage than the first buck voltage V_(OUTH). In one embodiment the first buck voltage V_(OUTH) is between 1 V and 3.3 V, while the second buck voltage V_(OUTL) is between 0.4 V and 0.9 V. In an exemplary embodiment, the clock generator 48 has a power input PWR coupled to the second converter output 60 so that the clock generator 48 is powered by the second buck voltage V_(OUTL). However, it is to be understood that the clock generator 48 can also be powered by the first buck voltage V_(OUTH) by coupling the power input PWR to the first converter output 56.

A mixed energy storage capacitor 62 is coupled between the output node 18 and a fixed voltage node 64 that is typically ground. The mixed energy storage capacitor 62 is charged with the voltage V_(MIX,OUT) and typically has a capacitance in the microfarad range. However, it is to be understood that other capacitance ranges for the mixed energy storage capacitor 62 are within the scope of the present disclosure.

In greater detail, the first active diode D_(ACT1) is made up of a first transmission gate transistor 66 having a first gate terminal 68, a first drain terminal 70 coupled to the first input node 16, and a first source terminal 72 coupled to the output node 18. A first clocked comparator 74 has a first comparator output 76 coupled to the first gate terminal 68, a first non-inverting input 78 coupled to the first input node 16, and a first inverting input terminal 80 coupled to the output node 18. A second transmission gate transistor 82 has a second gate 84 that receives an inverted version of a first comparator signal X1 that is output from the first comparator output 76, a second source terminal 86 coupled to the first input node 16, and a second drain terminal 88 coupled to the output node 18. A first parasitic diode D_(PAR1) has an anode coupled to the first input node 16 and a cathode coupled to the output node 18. The first parasitic diode D_(PAR1) provides an initial charging path between the first input node 16 and the output node 18.

The second active diode D_(ACT2) is made up of a third transmission gate transistor 90 having a third gate terminal 92, a third drain terminal 94 coupled to the second input node 20, and a third source terminal 96 coupled to the output node 18. A second clocked comparator 98 has a second comparator output 100 coupled to the third gate terminal 92, a second non-inverting input 102 coupled to the second input node 20, and a second inverting input terminal 104 coupled to the output node 18. The clocked nature of the first clocked comparator 74 and the second clocked comparator 98 provide enhanced energy efficiency because the first comparator 74 and the second clocked comparator 98 are only active at clock transitions.

The second active diode D_(ACT2) also includes a fourth transmission gate transistor 106 having a fourth gate terminal 108 that receives an inverted version of a second comparator signal X2 that is output from the second comparator output 100, a fourth source terminal 110 coupled to the second input node 20, and a fourth drain terminal 112 coupled to the output node 18. A second parasitic D_(PAR2) has an anode coupled to the second input node 20 and a cathode coupled to the output node 18. The second parasitic diode D_(PAR2) provides an initial charging path between the second input node 20 and the output node 18.

In greater detail, the first switching circuitry 30 has a first transistor M_(N1) having a first current terminal 114 coupled to the dynamic node 24, a second current terminal 116 coupled to the fixed voltage node 32, and a first control terminal 118 coupled to the control circuitry 34 such that in response to a first state of the control signal ϕ₁, the dynamic node 24 is coupled to the fixed voltage node 32, and in response to a second state of the control signal ϕ₁, the dynamic node 24 is decoupled from the fixed voltage node 32.

The first switching circuitry 30 also has a second transistor M_(P1) having a third current terminal 120 coupled to the dynamic node 24, a fourth current terminal 122 coupled to the second input node 20, and a second control terminal 124 coupled to the control circuitry 34 such that in response to the first state of the control signal ϕ₁, the dynamic node 24 is decoupled from the second input node 20, and in response to the second state of the control signal ϕ₁, the dynamic node 24 is coupled to the second input node 20 such that the first capacitor 22 and the second capacitor 26 are coupled in series.

In the exemplary embodiment of FIG. 1, the first transistor M_(N1) is a negative-channel metal oxide semiconductor (NMOS) device and the second transistor M_(P1) is a positive-channel metal oxide semiconductor (PMOS) device. As such, the first state of the control signal ϕ₁ is a positive voltage that is sufficient to turn the first transistor M_(N1) on and the second transistor M_(P1) off. In contrast, the second state of the control signal ϕ₁ is a lower voltage that is sufficiently close to zero volts to ensure that the first transistor M_(N1) is on and the second transistor M_(P1) is off.

In greater detail, the control circuitry 34 includes an OR gate 126 having a signal output node 128 that transmits that control signal ϕ₁ to the first control terminal 118 of the first transistor M_(N1) and the second control terminal 124 of the second transistor M_(P1). The OR gate 126 receives input from a re-startup controller 130 that is responsive to the master clock signal CLK and the voltage level (i.e., V_(PE),out) at the first input node 16. During transitions of the master clock signal CLK, the state of V_(PE),out, either charged or discharged, is transmitted to the OR gate 126 by the re-startup controller 130. The OR gate 126 also receives input from a clocked re-startup comparator 132 that compares a fraction of the first buck voltage V_(OUTH) with the second buck voltage V_(OUTL). A voltage divider 134 provides the fraction of the first buck voltage V_(OUTH). If the fraction of the first buck voltage V_(OUTH) is greater than the second buck voltage V_(OUTL), the clocked re-startup comparator 132 sends a high logic level to the OR gate 126, which in turn will drive the control signal ϕ₁ to a high logic level. Conversely, if the fraction of the first buck voltage V_(OUTH) is lower than the second buck voltage V_(OUTL), the clocked re-startup comparator 132 sends a low logic level to the OR gate 126, which in turn will drive the control signal ϕ₁ to the logic level of the re-startup controller 130.

FIG. 2 is another exemplary embodiment of the energy mixer 10 that includes a third energy harvester 136 having third voltage output 138 coupled to a fourth node 140, and a third active diode D_(ACT3) coupled between the fourth node 140 and the output node 18. A third capacitor 142 for storing energy gathered by the third energy harvester 136 is coupled between the fourth node 140 and a fifth node 144, which is typically ground. It is to be understood that the third energy harvester 136 can be an additional piezoelectric harvester or an additional thermoelectric generator harvester or any other type of energy harvester that converters one form of energy into electrical energy.

A second switching circuitry 146 has a third transistor M_(N2) having a fifth current terminal 148 coupled to the third node 28, a sixth current terminal 150 coupled to the fixed voltage node 32, and a third control terminal 152 coupled to the signal output node 128 of the control circuitry 34 such that in response to the first state of the control signal ϕ₁, the third node 28 is coupled to the fixed voltage node 32, and in response to a second state of the control signal ϕ₁, the third node 28 is decoupled from the fixed voltage node 32.

The second switching circuitry 146 also includes a fourth transistor M_(P2) having a seventh current terminal 154 coupled to the third node 28 and an eighth current terminal 156 coupled to the fourth node 140, and a fourth control terminal 158 coupled the to signal output node 128 of the control circuitry 34 such that in response to the first state of the control signal ϕ₁, the third node 28 is decoupled from the fourth node 140, and in response to the second state of the control signal ϕ₁, the third node 28 is coupled to the fourth node 140 such that the second capacitor 26 and the third capacitor 142 are coupled in series. It should be noted that in this exemplary embodiment depicted in FIG. 2, the first switching circuitry 30 in response to the second state of the control signal ϕ₁ will simultaneously couple the dynamic node 24 to the second input node 20 such that the first capacitor 22 will be coupled in series with the second capacitor 26. As a result, the second state of the control signal ϕ₁ will place all three of the first capacitor 22, the second capacitor 26, and the third capacitor 142 in series.

In the exemplary embodiment of FIG. 2, the third transistor M_(N2) is a NMOS device and the fourth transistor M_(P2) is a PMOS device. As such, the first state of the control signal ϕ₁ is a positive voltage that is sufficient to turn the third transistor M_(N2) on and the fourth transistor M_(P2) off. In contrast, the second state of the control signal ϕ₁ is a lower voltage that is sufficiently close to zero volts to ensure that the first transistor M_(N2) is on and the fourth transistor M_(P2) is off.

In greater detail, the third active diode D_(ACT3) also includes a fifth transmission gate transistor 160 having a fifth gate terminal 162, a fifth drain terminal 164 coupled to the fourth node 140, a fifth source terminal 166 coupled to the output node 18, a third clocked comparator 168 having a third comparator output 170 coupled to the fifth gate terminal 162, a third non-inverting input 172 coupled to fourth node 140, and a third inverting input terminal 174 coupled to the output node 18.

The third active diode D_(ACT3) also includes a sixth transmission gate transistor 176 having a sixth gate terminal 178 that receives an inverted version of a third comparator signal X3 that is output from the third comparator output 170, a sixth source terminal 180 coupled to the fourth node 140, a sixth drain terminal 166 coupled to the output node 18, a third parasitic diode D_(PAR3) having an anode coupled to the fourth node 140, and a cathode coupled to the output node 18.

It is to be understood that the embodiment of the energy mixer 10 depicted in FIG. 2 is only exemplary and that additional energy harvesters may be accommodated by including an energy storing capacitor like the third capacitor 142, switching circuitry like the second switching circuitry 146, and an active diode like the third active diode D_(ACT3) for each energy harvester. The additional energy harvesters, energy storage capacitors, switching circuitry, and active diodes are configured relative to the energy mixer 10 as described above.

Referring back to FIG. 1 and forward to FIGS. 3-6, a description of operation for the energy mixer 10 follows. In operation and at initial startup of the energy mixer 10, the first capacitor 22, the second capacitor 26, and the mixed energy storage capacitor 62 are not charged. As an energy harvester that has passive energy conversion such as the piezoelectric harvester 40 of the first energy harvester 12 begins to gather energy, the first capacitor 22 will charge to the voltage V_(PE,OUT). FIG. 3 is a simulation graph of voltage V_(PE,OUT) stored by a capacitor that stores energy gathered by a piezoelectric harvester 40 when the first switching circuitry 30 is inactive. During initial startup, charges will transfer through the parasitic diode D_(PAR1) from the first capacitor 22 to the mixed energy storage capacitor 62 because both the first transmission gate transistor 66 and the second transmission gate transistor 82 are off. At some point during the initial startup period, the voltage V_(MIX,OUT) will be high enough to activate both the first buck converter 50 and the second buck converter 52 to generate the first buck voltage V_(OUTH) and the second buck voltage V_(OUTL), respectively. Both the first buck voltage V_(OUTH) and the second buck voltage V_(OUTL) are used to drive loads such as an external static random access memory (SRAM) and an external central processing unit (CPU) (not shown). At least one of the first buck voltage V_(OUTH) and the second buck voltage V_(OUTL) are used to power the clock generator 48, which in turn activates the DC-DC converter 46 to begin boosting voltage generated by the TEG harvester 44 by providing the first clock signal CLK1 and the second clock signal CLK2 to the DC-DC converter 46. Refer to FIG. 4, which is a simulation graph of the voltage V_(TEG,OUT) stored by the second capacitor 26 when the first switching circuitry 30 is inactive. Notice that in this case, the second energy harvester 14 is providing full power before 5 milliseconds and at around 5 milliseconds the first energy harvester 12 begins the initial charging of the mixed energy storage capacitor 62.

FIG. 5 is a simulation graph of the output voltage V_(MIX,OUT) at the output node 18 when the first switching circuitry 30 is inactive. This graph represents normal operation for the energy mixer 10 wherein the output voltage V_(MIX,OUT) is at a high enough voltage level to generate both V_(OUTL) and V_(OUTH). As a result of normal operation, the control signal ϕ1 will be at a logic 1 voltage level.

FIG. 6 is a simulation graph of the control signal ϕ1 generated by the energy mixer 10 when the first switching circuitry 30 is inactive. During normal operation, the first switching circuitry 30 is inactive and the control signal ϕ1 remains at a constant logic 1 voltage level to ensure that the first capacitor 22 and the second capacitor 26 are separated and not placed in series while both the first energy harvester 12 and the second energy harvester 14 are both substantially contributing energy to the output voltage V_(MIX,OUT).

In contrast, if one or the other of the first energy harvester 12 and the second energy harvester 14 has a substantial drop in energy contribution to the output voltage V_(MIX,OUT), the clocked re-startup comparator 132 is triggered to output a logic 0. This action indicates that the output voltage V_(MIX,OUT) is not at a voltage level high enough to generate to the first buck output voltage V_(OUTH), but that the second buck output voltage V_(OUTL) is still being generated. As a result, the first switching circuitry 30 becomes active and the control signal ϕ1 alternates between logic 1 and a logic 0.

FIG. 7 is a simulation graph of the control signal ϕ1 generated by the energy mixer 10 when the first switching circuitry 30 is active. During a logic 0, the first capacitor 22 and the second capacitor 26 are placed in series by the first switching circuitry 30 being active. As a result, the voltage V_(PE),out at the first input node 16 and the voltage V_(TEG,OUT) at the second input node 20 are summed together, which provides a high enough voltage level for the first active diode D_(ACT1) to pass charges to the mixed energy storage capacitor 62. Once the charges are transferred, the first active diode D_(ACT1) is turned off and the re-startup controller 130 forces the control signal ϕ₁ to be a logic 1 so that the first capacitor 22 and the second capacitor 26 are separated and are recharged by the first energy harvester 12 and the second energy harvester 14, respectively. However, the control signal ϕ₁ will alternate between a logic 1 and a logic 0 until the output voltage V_(MIX,OUT) is at a voltage level high enough to resume normal operation by once again generating the first buck voltage V_(OUTH).

FIG. 8 is a simulation graph of the voltage V_(PE,OUT) stored by the first capacitor 22 that stores energy gathered by the piezoelectric harvester 40 coupled to the energy mixer 10 when the first switching circuitry 30 is active. FIG. 9 is a simulation graph of the voltage V_(TEG,OUT) stored by the second capacitor 26 that stores energy gathered by the TEG harvester 44 coupled to the energy mixer 10 when the first switching circuitry 30 is active. FIG. 10 is a simulation graph of the output voltage V_(MIX,OUT) that eventually charges to the level of V_(PE,OUT)+V_(TEG,OUT), which in this exemplary case is approximately equal to 1.4V.

The energy mixer 10 of FIG. 1 and FIG. 2 offers an integrated solution having an initial startup mode using a passive energy harvester, efficient energy mixing, and continued operation when energy levels gathered by one or more energy harvesters drops to a level that could not ordinarily allow continuous operation and thereby avoid energy wasting restarts.

Those skilled in the art will recognize improvements and modifications to the preferred embodiments of the present disclosure. All such improvements and modifications are considered within the scope of the concepts disclosed herein and the claims that follow. 

What is claimed is:
 1. A method of operating an energy mixer having switching circuitry to combine energy harvested from a plurality of energy harvesters comprising: charging energy storing capacitors coupled to the switching circuitry with the energy harvested by the plurality of energy harvesters, each one of the plurality of energy harvesters being coupled between a fixed voltage node and a respective one of the energy storing capacitors; mixing the energy harvested by coupling the energy storing capacitors corresponding to respective ones of the plurality of energy harvesters to an output node by way of respective switches of the switching circuitry to provide an output voltage; comparing the output voltage to a predetermined voltage range by way of control circuitry in control of the switching circuitry to determine if the output voltage is within or below the predetermined voltage range; coupling the energy storing capacitors in series by way of the switching circuitry when the control circuitry determines that the output voltage is below the predetermined voltage range; and decoupling of the energy storing capacitors out of series by way of the switching circuitry when the control circuitry determines the output voltage is within the predetermined voltage range.
 2. The method of operating the energy mixer of claim 1 wherein mixing harvested energy includes receiving electrical energy from the plurality of energy harvesters that convert at least two different forms of energy to electrical energy.
 3. The method of operating the energy mixer of claim 2 wherein at least one of the plurality of energy harvesters is a piezoelectric harvester.
 4. The method of operating the energy mixer of claim 3 wherein a rectifier configured to convert an alternating current (AC) voltage to a direct current (DC) voltage is coupled between the piezoelectric harvester and the energy mixer.
 5. The method of operating the energy mixer of claim 2 wherein at least one of the plurality of energy harvesters is a thermoelectric harvester.
 6. The method of operating the energy mixer of claim 5 wherein a DC-DC converter configured to convert DC voltage to a higher voltage level is coupled between the thermoelectric harvester and the energy mixer.
 7. The method of operating the energy mixer of claim 2 wherein at least one of the plurality of energy harvesters is a piezoelectric harvester and at least one other of the plurality of energy harvesters is a thermoelectric harvester.
 8. The method of operating the energy mixer of claim 7 wherein a rectifier configured to convert an AC voltage to a DC voltage is coupled between the piezoelectric harvester and the energy mixer and a DC-DC converter configured to convert DC voltage to a higher voltage level is coupled between the thermoelectric harvester and the energy mixer.
 9. The method of operating the energy mixer of claim 1 wherein the energy mixer includes an active diode for each one of the plurality of energy harvesters, wherein each active diode is configured to transfer energy from energy storing capacitors through the energy mixer to an output node with a minimum voltage drop.
 10. The method of operating the energy mixer of claim 9 wherein a first voltage converter is coupled to the output node and is configured to convert the output voltage at the output node to a first converted voltage and convert the output voltage to a second converted voltage.
 11. A method of operating an energy mixer having switching circuitry to combine energy harvested from a plurality of energy harvesters comprising: charging energy storing capacitors coupled to the switching circuitry with the energy harvested by the plurality of energy harvesters; mixing the energy harvested by coupling the energy storing capacitors corresponding to respective ones of the plurality of energy harvesters to an output node by way of respective switches of the switching circuitry to provide an output voltage; converting the output voltage to a first converted voltage by way of a first voltage converter; converting the output voltage to a second converted voltage by way of a second voltage converter; comparing the second converted voltage to a predetermined fraction of the first converted voltage by way of control circuitry in control of the switching circuitry to determine if the predetermined fraction of the first converted voltage is above or below the second converted voltage; coupling the energy storing capacitors in series by way of the switching circuitry when the control circuitry determines that the predetermined fraction of the first converted voltage is below the second converted voltage; and decoupling of the energy storing capacitors out of series by way of the switching circuitry when the control circuitry determines the predetermined fraction of the first converted voltage is above the second converted voltage.
 12. The method of operating the energy mixer of claim 11 wherein mixing harvested energy includes receiving electrical energy from the plurality of energy harvesters that convert at least two different forms of energy to electrical energy.
 13. The method of operating the energy mixer of claim 12 wherein at least one of the plurality of energy harvesters is a piezoelectric harvester.
 14. The method of operating the energy mixer of claim 13 wherein a rectifier configured to convert an alternating current (AC) voltage to a direct current (DC) voltage is coupled between the piezoelectric harvester and the energy mixer.
 15. The method of operating the energy mixer of claim 12 wherein at least one of the plurality of energy harvesters is a thermoelectric harvester.
 16. The method of operating the energy mixer of claim 15 wherein a DC-DC converter configured to convert DC voltage to a higher voltage level is coupled between the thermoelectric harvester and the energy mixer.
 17. The method of operating the energy mixer of claim 12 wherein at least one of the plurality of energy harvesters is a piezoelectric harvester and at least one other of the plurality of energy harvesters is a thermoelectric harvester.
 18. The method of operating the energy mixer of claim 17 wherein a rectifier configured to convert an AC voltage to a DC voltage is coupled between the piezoelectric harvester and the energy mixer and a DC-DC converter configured to convert DC voltage to a higher voltage level is coupled between the thermoelectric harvester and the energy mixer.
 19. The method of operating the energy mixer of claim 11 wherein the energy mixer includes an active diode for each one of the plurality of energy harvesters, wherein each active diode is configured to transfer energy from energy storing capacitors through the energy mixer to an output node with a minimum voltage drop.
 20. The method of operating the energy mixer of claim 11 wherein the first voltage converter is a buck converter and the second voltage converter is a buck converter. 